In recent years, efforts have actively been made toward environmental issues and energy issues for the sustainable society. Expectations of thermoelectric converter elements have grown under such circumstances.
This is because heat is the most common energy source that is available from various media, such as body temperature, sunlight, engines, and industrial exhaust heat.
Therefore, thermoelectric converter elements are expected to become more important in future for efficiency enhancement in energy use for a low-carbon economy or for applications of power supply to ubiquitous terminals, sensors, or the like.
Heretofore, a bulk thermoelectric converter element comprising a thermocouple module structure assembled by processing and bonding a sintered compact of a thermoelectric semiconductor such as Bi2Te3 has commonly been used as a structure of a thermoelectric converter element. However, a thin-film thermoelectric element comprising a module produced by depositing a thin film of a thermoelectric semiconductor on a substrate by a sputtering method or the like has progressed in development and attracted attention.
Examples of the advantages of such a thin-film thermoelectric converter element are given as follows: (1) A thin-film thermoelectric converter is small in size and light in weight. (2) A collective deposition for a large area can be achieved by sputtering, coating, printing, or the like. Thus, the productivity is high. (3) Cost can be reduced by using an inexpensive substrate. (4) A flexible thermoelectric converter element can be obtained by using a highly flexible substrate.
Here, thin-film thermoelectric converter elements have heretofore been produced by coating or printing. For example, according to Patent Literature 1, powdered Bi2Te3 is mixed with a binder into paste, which is applied onto a substrate by a screen printing method or the like so as to form a thermoelectric element pattern. Furthermore, according to Patent Literature 2, an ink including a thermoelectric semiconductor material and an electrode material is pattern-printed by an ink jet method so as to form a thermoelectric element. Moreover, according to Patent Literature 3, an organic semiconductor is used as a thermoelectric material, and a thermoelectric element is formed by a printing process.
However, there has been a problem that the aforementioned thin-film thermoelectric element is so thin that it has difficulty in generating and holding a temperature difference between a front face and a rear face of the thin film. Specifically, in most of power generation applications, a temperature difference (temperature gradient) is imparted in a direction perpendicular to a thin-film surface comprising a thermoelectric material, so that thermoelectric conversion is performed. As the film thickness of a thin film of a thermoelectric semiconductor is reduced, thermal insulation (thermal resistance) becomes insufficient. Therefore, it becomes difficult to maintain a temperature difference between a front face and a rear face of the thin film of the thermoelectric semiconductor. Alternatively, a temperature difference is mostly generated between a front face and a rear face of a substrate, rather than a front face and a rear face of the thin film of the thermoelectric semiconductor. Accordingly, efficient power generation cannot be achieved.
In order to improve the thermal insulation property, one of the following two solutions may be taken: (1) The film thickness of a thermoelectric semiconductor film is increased (for example, to at least several times 10 μm). (2) The thermal conductivity of a thermoelectric semiconductor is reduced.
However, in the case of the solution (1), it becomes difficult to pattern and produce a thermocouple structure by a coating process, a printing process, or the like if the film thickness of a thermoelectric semiconductor film increases. Therefore, the productivity decreases. Thus, a trade-off arises between increased conversion efficiency and reduced cost productivity.
Furthermore, in the case of the solution (2), a material having a lower thermal conductivity tends to have a lower electric conductivity. Additionally, a thermoelectric material having a high electric conductivity is required for conventional thermoelectric generation. In view of those facts, a trade-off still arises between the electric conductivity and the thermal conductivity. Therefore, there is a limit in reduction of the thermal conductivity.
Meanwhile, in recent years, there has been discovered the spin Seebeck effect, which generates electron spin currents when a temperature gradient is applied to a magnetic material.
Patent Literature 4 and Non-Patent Literatures 1 and 2 disclose a thermoelectric converter element based upon the spin Seebeck effect and illustrate a structure in which currents of angular momentum (spin currents) caused by the spin Seebeck effect are derived as an electric current (electromotive force) by the inverse spin Hall effect.
For example, a thermoelectric converter element disclosed in Patent Literature 4 includes a ferromagnetic metal film deposited by a sputtering method and a metal electrode. With this configuration, when a temperature gradient is applied in a direction parallel to a surface of the ferromagnetic metal film, spin currents are induced along the temperature gradient by the spin Seebeck effect. The induced spin currents can be derived as an electric current to the exterior of the thermoelectric converter element by the inverse spin Hall effect of the metal electrode that is brought into contact with the ferromagnetic metal. Thus, a temperature difference power generation that derives electric power from heat can be achieved.
Furthermore, a thermoelectric converter element disclosed in Non-Patent Literatures 1 and 2 is formed of a magnetic insulator and a metal electrode.
Specifically, in Non-Patent Literature 1, there has been reported a thermoelectric conversion in which a temperature gradient is arranged in parallel to a surface of the magnetic insulator (in-plane temperature gradient) as with Patent Literature 4.
Moreover, Non-Patent Literature 2 exemplifies thermoelectric conversion with an arrangement of a temperature gradient perpendicular to a plate surface of the magnetic insulator having a thickness of 1 mm (perpendicular-plane temperature gradient).
With use of the spin Seebeck effect, a complicated thermocouple structure is not required, unlike a conventional thermoelectric converter element using a thermocouple module configuration. Therefore, the aforementioned problems relating to the arrangement patterning may be solved, and a thin-film thermoelectric converter element that can readily increase its area at a low cost may be obtained.
Furthermore, in a thermoelectric converter element using the spin Seebeck effect, an electrically conductive portion (electrode) and a thermally conductive portion (magnetic material) can be designed independently of each other. In theory, a structure having a high electric conductivity (low ohmic loss) and a low thermal conductivity (capable of holding a temperature difference between a front face and a rear face thereof) can be implemented.
For example, when an insulation material is used for a magnetic material as in Non-Patent Literatures 1 and 2, heat conduction through electrons can completely be inhibited. Therefore, development of a high-performance thermoelectric converter element that can achieve sufficient thermal insulation with a thin-film material is anticipated.
Patent Literature 5 discloses the following structure. Two metal electrodes are provided on a magnetic and dielectric layer. Spin currents induced in one of the electrodes by signal currents are exchanged with spins in the magnetic and dielectric layer to generate spin currents of spin waves and propagate them through the magnetic and dielectric layer. The spin currents of spin waves and pure spin waves are exchanged with each other at an interface between the other electrode and the magnetic and dielectric layer to thereby generate signal power on the other electrode. Thus, signal currents are transmitted between the two electrodes (Patent Literature 5).